Hi guys got a bit of a dilemma, really need some help please

[Sonnypaul]

Need some advice on the best way to set out my common rafters for a pitch roof I'm doing, the only thing I don't know is the correct way to set out, I know how to cut the birds mouth and use the angle for this roof which is 17.5 degrees, the wall plate on the new block work is on and strapped but the wall plate on the existing house wall is not on yet, do I cut my common rafters and then fix the wall plate underneath them? I just don't know how high to put the wall plate of the floor, Oh and if I bird mouth the seat cut on the commons do I have to bird mouth the top cut aswell, I just don't want to change the angle or muck it up

Hope I explained it ok lol not easy typing it as to saying it out Loud!

Would love to see some comments

Thank you.

 

[^woody^]

You need to notch both ends of the rafter, and the wall bearer needs to be fixed before the rafters

Either mark out the pitch with string and then fit the bearer plate, or notch the first rafter, put it on temporary and then fix the bearer beneath before the other rafters

Then before cutting all the rafters to the same template, check that the wall plate is level and parallel to the wall, or else all the notches and rafter lengths will be slightly different

 

[noseall]

You would typically fit the two end rafters then offer the pole plate up underneath into the (notched) ends. Then fix the pole plate.

We have started fitting the middle rafter also to determine whether there ie any bow in the wall, whether that be bowing in or out. If the wall bows in then you can pack the plate. If it bows out then you would need to shorten the rafters to suit otherwise you tend to force the pole plate upwards or the wall plate outwards.

 

[Sonnypaul]

You need to notch both ends of the rafter, and the wall bearer needs to be fixed before the rafters

Either mark out the pitch with string and then fit the bearer plate, or notch the first rafter, put it on temporary and then fix the bearer beneath before the other rafters

Then before cutting all the rafters to the same template, check that the wall plate is level and parallel to the wall, or else all the notches and rafter lengths will be slightly different

Hi woody thank you for the reply.

so just to re-cap

ill cut my timber at 17.5 degrees on one end (house wall plate) then offer it up, then mark my seat cut for the birds mouth on the new block work wall plate ( 1/3 of the timber removed yes? ) followed by putting a timber either end to check for level and then put the wall plate underneath the two timbers and if all is good use one of the timbers as a template.


And one more thing woody, if i only bird mouthed the bottom cut and not the top would that affect my angle?

thanks again

 

[theoldun]

Roofs are so easy once you know your maths.
On a 17.5 degree pitch, your seat cut on birds mouth is 17.5 degrees and your top plumb cut and heel cut to birds mouth is 72,5 degrees.
Every pitch has its own secant. The secant is the length of rafter for every metre of span, For 17.5 degree the secant is 1048.
Your span is the measure from existing house wall to outside of wall plate. Therefore if your span is 3000, then total secant (top plumb cut to birds mouth plumb cut) is 3144.If span was 2700 then total secant would be 2829.
Every pitch has its own rise, The rise on 17.5 is 315 for every metre of span Therefore if span is 3000, the rise from top of plate to top of rafter is 945. If span was 2700, then rise would be 859.
If using 100 to 150 depth timbers birds mouth is two thirds down heel plumb cut.
Birds beak top plumb cut ditto.
Read it through couple times and you will understand, if not then come back. Just read your last post. Plumb cut is 72.5 not 17.5
Regards oldun

 

[Sonnypaul]

Roofs are so easy once you know your maths.
On a 17.5 degree pitch, your seat cut on birds mouth is 17.5 degrees and your top plumb cut and heel cut to birds mouth is 72,5 degrees.
Every pitch has its own secant. The secant is the length of rafter for every meter of span, For 17.5 degree the secant is 1048.
Your span is the measure from existing house wall to outside of wall plate. Therefore if your span is 3000, then total secant (top plumb cut to birds mouth plumb cut) is 3144.If span was 2700 then total secant would be 2829.
Every pitch has its own rise, The rise on 17.5 is 315 for every meter of span Therefore if span is 3000, the rise from top of plate to top of rafter is 945. If span was 2700, then rise would be 859.
If using 100 to 150 depth timbers birds mouth is two thirds down heel plumb cut.
Birds beak top plumb cut ditto.
Read it through couple times and you will understand, if not then come back. Just read your last post. Plumb cut is 72.5 not 17.5
Regards oldun

Thank you theoldun, will need to read this many times, this has just confused the hell out of me haa, once you know you'll know it for life aye.

 

[^woody^]

if i only bird mouthed the bottom cut and not the top would that affect my angle?

No it wont affect the angle of the rafter as all it will mean is that the bearer plate on the house wall will be positioned a little lower

The top of the rafter should have a notch too as this not only acts to put the load down vertically on a wider bearing, but it stops the tenancy for the rafter to try and slide and put loading onto the extension wall and push it outwards